In comparison with an ingot alloy of a metal produced by casting, an aluminum powder alloy has an advantage of capable of making a crystal grain fine, adding other elements more, and further dispersing a reinforcing material uniformly. Therefore, in recent years the usage of the aluminum powder alloy is spreading into such a construction and civil usage, including an automobile, a vehicle, and an aircraft.
Conventionally, an aluminum powder alloy is produced, in a state of being tentatively molded by cold isostatic pressing, hot pressing, or the like; in a state of the tentatively molded powder alloy being accommodated in a container; or in a state of being further sealed in a can or the like and being pressure sintered, by being further hot rolled plastically, thereby being molded into a predetermined shape, and being pressure sintered.
Whereas if a reinforcing material and the like are contained in an aluminum powder alloy, the composite becomes brittle; therefore, according to the conventional processing method, there is a problem that it is difficult to mold the aluminum powder alloy into a desired shape.
Therefore, the applicants suggested a method, which is described in the pamphlet of WO 2006/070879, of producing a rolled material ensuring a plastic workability by hot rolling metal powder after the metal powder is current pressure sintered wholly together with a metal container in a state of the metal powder being accommodated in the metal container, and the method has come into practical use.
In the method of producing such a rolled material, there is used in some instances an object formed like a box, as shown in FIGS. 6A and 6B, by combining four metal plates 111, 112, 113, 114 by means of welding and the like as a metal container 101 accommodating metal powder 102; thereby forming a rectangular frame member 110; and closing an opening of the frame member 110 with lid members 115, 115 consisting of a rectangular metal plate.
However, with respect to the conventional metal container 101, the metal plates 111, 112, 113, 114 are joined at corners where a large force acts in rolling; therefore, when an aluminum powder alloy is rolled, a breakage occurs at the corners (joints) of the frame member 110 and the rolling is not achieved in some instances.
In addition, as shown in FIGS. 6C and 6D, for the purpose of preventing a breakage at a joint between metal plates configuring a frame member, there is used in some instances a frame member 120 integrally molded by doing away with the joint between the metal plates. Whereas, in order to integrally mold the frame member 120, there is a problem that production cost runs up; for example, it is necessary to prepare a large extrusion machine.